A predecessor to Coordinated Multipoint Transmission/Reception (COMP), then denoted Distributed Antenna System (DAS), was originally introduced for coverage improvement in indoor wireless communications, as described by A. A. M. Saleh, A. J. Rustako Jr., and R. S. Roman, in a paper titled “Distributed antennas for indoor radio communications,” published in the IEEE Transactions on Communications, vol. 35, no. 12, pp. 1245-1251, 1987. Their approach was directed towards transmission to a single user through a discrete implementation of a leaky feeder. The notion of COMP in contrast enables multiple data streams to be transmitted over an interconnected network of radioheads (or basestations) where the different signals representative of the multiple data streams may be controlled by weightings and distributed to the different radio heads. The idea of COMP may be used in downlink as well as uplink. In this invention we are concerned with downlink only. Recent studies indicate that COMP can provide not only coverage improvement but also capacity enhancement, as described by J. Gan et al., in a paper titled “On sum rate and power consumption of multi-User distributed antenna system with circular antenna layout,” published in the EURASIP Journal on Wireless Communications and Networking, vol. 2007, Article ID 89780.
Techniques exploring the advantages of COMP can be classified into two categories: Single-User COMP (SU-COMP) and Multi-User COMP (MU-COMP). SU-COMP techniques attempt to improve the link quality for a single user by means of spatial multiplexing, or spatial diversity. However, SU-COMP techniques can not manage the mutual interference among users. Accordingly, Radio Resource Management (RRM) schemes are needed for geographically separated users that are using the same time/frequency resources. The reuse distance restricts the capacity increase of SU-COMP.
MU-COMP techniques jointly process signals to/from multiple users and attempt to improve the overall system performance. MU-COMP is quite similar to Multi-User Multiple-Input Multiple-Output (MU-MIMO) systems. Accordingly, techniques developed for MU-MIMO system, such as Zero-Forcing (ZF) beamforming and Dirty Paper Coding (DPC), can be directly applied to MU-COMP. Some of these techniques are described by G. J. Foschini et al., in a paper titled “The value of coherent base station coordination,” published in the Proceedings of the 39th Annual Conference on Information Sciences and Systems (CISS '05), March 2005.
MU-COMP techniques can achieve the capacity limit provided by a COMP, as there is no need to separate users in time/frequency to avoid mutual interference, as in SU-COMP. However, for the downlink transmission, the transmitter needs to know all channel state information (CSI), which is impractical to implement. For MU-COMP, the direct application of traditional MU-MIMO techniques has two main drawbacks.
First, all channel elements are fed back, i.e. transmitted in uplink from the UE. This generates excessive uplink overhead and reduces the available resources for other desired uplink traffic. One example method of MU-COMP for downlink where the full channel knowledge is available (such as via feedback) at the transmitter is in zero forcing (ZF) beamforming, with the beamweight matrix W=HH(HHH)−1. In this case the received signal can be expressed asy=H(HH(HHH)−1)x+n=x+n  (1)As equation (1) indicates, ZF beamforming not only eliminates the interferences, but also normalizes the channel response of the desired signal to be 1. Since the UE can adjust the phase of the received signal before detection, such transmitter-side normalization is not necessary. Further, the amplitude normalization to one for every user is not always desired since different link qualities and rates may be desired. Hence, the transmit power may also be set individually for each, after ZF, interference free link.
Second, the characteristics of MU-COMP channel are not fully explored. More specifically, for MU-MIMO, the channel between the transmitter and the receiver can be modeled using Independent and Identically Distributed (IID) random variables. However, for MU-COMP, since the antennas are geographically distributed, the channel elements between each transmitter and a receiver are not identically distributed. In most cases, the channel response of an undesired signal is much weaker than that of the desired signal.